Marz,
Probably only works for normal light photography, not infra-red -
which is what many speed cameras are using now. The spray may be legal
to purchase, but it's illegal to use it on number plates.
Also, to deliberately obscure one's number plate will result in a BIG
fine (I hear it's near $1000), plus loss of quite a few points.

Tom Torok
Mobile Motorcycle Detailing
tom2rok at optusnet.com.au
0412 355 152